1. Field of the Invention
The present invention relates to a radio communication system which is constituted of a base station equipped with a variable beam-pattern array antenna such as an adaptive array antenna, and a control station connected to the base station via an optical fiber and which is provided with a function of controlling the variable beam-pattern array antenna on the side of the control station.
2. Related Background Art
Much attention has been paid to a radio on fiber (ROF) technique of connecting a base station and a control station for mobile communication represented by cellular phones and intelligent transport systems (ITS) to each other by an optical fiber in order to perform signal transmission. According to the ROF technique, a radio signal is transmitted from the base station to the control station via an optical fiber, and a modulator/demodulator, a controller and the like are collectively contained in the control station in order to simplify and miniaturize a constitution of base station. Therefore, it is possible to arrange a plurality of base stations along a road, in an underground shopping center, in a tunnel, and the like.
Moreover, in order to solve problems such as tightness of a frequency band in the base station, and an interference wave, an adaptive antenna capable of varying directivity has been noted. The adaptive antenna is provided with an array antenna having a plurality of antenna elements, and the radiation beam-pattern of the antenna can be changed by transmitted signals transmitted from the respective antenna elements.
A beam calculation circuit in the control station derives a radiation pattern of a radio signal to a subscriber from the base station, and the radiation pattern of the radio signal to the base station from the subscriber, and changes the radiation beam-pattern of the adaptive antenna in an adaptive manner in accordance with movement and position of the subscriber.
Several reports on a radio communication system, in which the base station is provided with this type of adaptive antenna and which is connected to the control station by using the ROF technique, have been published (e.g., Japanese Patent Application Laid-open No. 145286/1998).
FIGS. 1 and 2 are block diagrams schematically showing a constitution of the radio communication system utilizing the ROF technique. In consideration of a transmission/reception function as the radio communication system, it is most important to transmit the radio signal received by each antenna element to the control station from the base station while a relative phase difference and a relative intensity difference are maintained.
Therefore, in the conventional system, a transmitted/received signal of each antenna element is converted to a optical signal, and then transmitted between the base station and the control station by multiplexing a wavelength, or by allotting a specific optical fiber to each antenna element.
In the conventional system utilizing the ROF technique, however, as shown in FIGS. 1 and 2, a pair of an electric/optical converter and an optical/electric converter are allotted to each antenna element line, and the base station and control station require pairs of optical transmitter/receiver by the number of antenna elements.
Therefore, elements constituting optical transmitters of the base station and the control station largely increase, the constitution is complicated and an apparatus becomes large-sized. Moreover, when wavelength multiplexing transmission is performed, an optical multiplexer, an optical branching filter, a wavelength control function of a light source, and other constitutions are further necessary.
On the other hand, if the optical fiber is provided by each antenna element, the number of optical fibers for connecting the base station to the control station largely increases, and accordingly the constitutions of optical transmitters such as the optical/electric converter and the electric/optical converter become complicated and large-sized.
Thus, in the conventional radio communication system, provided with the adaptive antenna, for utilizing the ROF technique to perform the signal transmission, since there are many constituting elements for the optical transmitter, the constitution is complicated, it is difficult to miniaturize the base station and control station, and cost cannot be reduced.